1. Field of the Invention
The present invention relates to a direct thermal printer, and more particularly to a direct thermal printer wherein the whole portion of a thermosensitive color recording medium is optically fixed. The present invention also relates to an optical fixing device for use in the direct thermal printer.
2. Related Art
There are generally two types of thermal printers: one type is for direct thermal recording using thermosensitive recording media and the other type is for thermal transfer recording including wax thermal transfer recording and thermal dye transfer recording. The direct thermal printers print images directly on thermosensitive recording media, so that no waste, such as a dye transfer sheet, is produced. Furthermore, the construction of the direct thermal printer is simpler and thus the running cost of the direct thermal printer is lower than that of the thermal transfer printer.
Because of these preferable features, the direct thermal printers tend to be used more frequently. For example, most facsimile transmitters are provided with monochromatic direct thermal printers. Recently, a thermosensitive color recording medium, has been suggested, for example, in Japanese Laid-open Patent Application 61-213169, which has, for example, three thermosensitive coloring layers for developing magenta, cyan and yellow colors formed on one another, so that full-color images can be recorded thereon by using direct thermal printers.
In the thermosensitive color recording medium, thermal recording is sequentially performed from the upper or outer most coloring layer to the lower or inner most coloring layer. After recording each of the coloring layers, electromagnetic rays having a wave length range which is specific to each coloring layer are projected onto the recording medium, for optically fixing the just-recorded coloring layer, so that the coloring layer is not repeatedly recorded during the following thermal recording process.
For example, when the yellow recording layer is disposed at the outermost of the coloring layers, and the second layer disposed under the yellow recording layer is the magenta recording layer, the yellow recording layer contains a diazonium salt compound which is optically decomposed by near ultraviolet rays having a wave length of about 420 nm, while the magenta recording layer contains a diazonium salt compound which is optically discomposed by ultraviolet rays having a wave length of about 365 nm. Thereby, the yellow recording layer is optically fixed by using the near ultraviolet rays of about 420 nm before thermal recording the magenta recording layer, and the magenta recording layer is optically fixed by using the ultraviolet rays of about 365 nm.
Because of the diazonium salt compounds, the thermosensitive recording medium is colored yellow. After being optically fixed, because the diazonium salt compounds have been discomposed, the thermosensitive recording medium becomes white.
Meanwhile, it is known in the thermal printer to use a clamp member for holding the recording medium, such as a sheet of thermosensitive color recording paper, onto a supporting material, such as a platen drum. The clamp member presses a portion of the thermosensitive color recording paper, mostly, the leading edge thereof, so as to prevent the thermosensitive color recording paper from slipping out from the platen drum while being rotated.
Because the portion pressed by the clamp member is not exposed to the electromagnetic rays for optical fixing, there has been a problem that the portion of the thermosensitive color recording paper remains yellow even after the completion of the recording and fixing processes.
On the other hand, it is conventional to use a linear lamp having a circular section for optical fixing. The linear lamp is disposed downstream of a thermal head in a direction parallel to a rotary shaft of the platen drum. However, the conventional linear lamp is inconvenient for exchange, because sockets for attaching the linear lamp are mounted to a pair of frames for supporting the rotary shaft of the platen drum. Accordingly, it is necessary to open a casing of the thermal printer when exchanging the lamp.